Elevated serotonin receptor 2A signaling restores learning and memory in a Fragile X syndrome model

Serotonin (5-hydroxytryptamine, 5-HT) has central roles enabling learning and memory, particularly via serotonin receptor 2A (5-HT_2AR) signaling. Drosophila Fragile X syndrome model (dfmr1 null mutant) studies reveal impaired learning and memory, which may reflect serotonergic signaling deficits. Here, we use classical olfactory T-maze conditioning to assess behavior, combined with imaging to assess 5-HT and 5-HT_2AR levels within the underlying Mushroom Body (MB) brain circuitry. Null dfmr1 mutants exhibit learning and memory deficits that are corrected by elevating 5-HT signaling via 1) overexpression of the serotonin biosynthetic enzyme tryptophan hydroxylase (Trhn) or 2) knockdown of the serotonin reuptake transporter (SERT). Direct comparisons reveal both Trhn and SERT manipulations equally restore learning and memory in dfmr1 null mutants. 5-HT_2AR levels in the MB circuit are reduced relative to controls in dfmr1 mutants, and 5-HT_2AR RNAi phenocopies dfmr1 null behavioral deficits, suggesting these phenotypes are primarily caused by the loss of 5-HT_2AR signaling. Consistently, 5-HT_2AR overexpression in dfmr1 nulls restores normal learning and memory compared to controls. These findings suggest loss of 5-HT_2AR signaling causes learning and memory deficits in this Fragile X syndrome model, and that rectifying this signaling impairment can restore learning and memory, providing a framework for serotonergic intervention strategies.